W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
The dedicated channel (DCH) for downlink and uplink and the downlink shared channel (DSCH) have been defined in Release 99 and Release 4. In the following years, the developers recognized that for providing multimedia services—or data services in general—high speed asymmetric access had to be implemented. In Release 5 the high-speed downlink packet access (HSDPA) was introduced. The new high-speed downlink shared channel (HS-DSCH) provides downlink high-speed access to the user from the UMTS Radio Access Network (RAN) to the communication terminals, called user equipments in the UMTS specifications.
HSDPA is based on techniques such as fast packet scheduling, adaptive modulation and hybrid ARQ (HARQ) to achieve high throughput, reduce delay and achieve high peak data rates.
Hybrid ARQ Schemes
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which are combined with Forward Error Correction (FEC), called Hybrid ARQ. If Cyclic Redundancy Check (CRC) detects an error, the receiver requests the transmitter to send additional bits or a new data packet. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit will be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ may be defined.
In HARQ Type I the erroneous data packets received, also called PDUs (Packet Data Unit) are discarded and new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU. Using HARQ Type II the erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Finally, HARQ Type II is almost the same packet retransmission scheme as Type II and only differs in that every retransmitted PDU is self-decodable. This implies that the PDU is decodable without the combination with previous PDUs. In case some PDUs are so heavily damaged that almost no information is reusable self decodable packets can be advantageously used.
Packet Scheduling
Packet scheduling is a resource management algorithm used for allocating transmission opportunities and transmission formats to the users' communication terminals (UEs) admitted to a shared medium as a radio channel Packet scheduling methodologies may be generally characterized by the following:                The scheduling granularity which may be defined as a time period in which data transmissions of the users are scheduled ahead in time. Finer granularities may require higher computational complexity of the respective scheduling algorithms. Typical scheduling granularity for Universal Mobile Telecommunication System (UMTS) High Speed Downlink Packet Access (HSDPA) is equal to 2 ms, the so-called Time Transmission Interval (TTI).        The serving order which may refer to the order in which the users' communication terminals are served. It is also referred to as the scheduling algorithm. The two typical implementations of the scheduling algorithm are round robin, in which a cyclically uniform allocation of transmission opportunities is provided to the users, and max C/I aiming to maximize the throughput by allowing only those users access to the shared medium that currently have the best instantaneous channel quality.        The allocation method which may be a set of criteria for allocating transmission formats. In UMTS HSDPA a typical allocation method comprises current channel condition, Quality of Service (QoS) requirements and amount of packets waiting to be transmitted. These criteria are monitored per flow of a particular user i.e. per flow of a particular UE.UMTS Architecture        
The high level R99/4/5 architecture of Universal Mobile Telecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, available from http://www.3gpp.org). The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the UEs. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called Iu connection with the Core Network (CN) 101. When required, a Drift RNS (D-RNS) supports the Serving RNS (S-RNS) by providing radio resources. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used.
Evolved UTRAN Architecture
Currently, the feasibility study for UTRAN Architecture Evolution from the current R99/4/5 UMTS architecture is ongoing (see 3GGP TSG RAN WG3: “Feasibility Study on the Evolution of the UTRAN Architecture”, available at http://www.3gpp.org). Two general proposals for the evolved architecture (see 3GGP TSG RAN WG3, meeting #36, “Proposed Architecture on UTRAN Evolution”, Tdoc R3-030678 and “Further Clarifications on the Presented Evolved Architecture”, Tdoc R3-030688, available at http://www.3gpp.org) have emerged. The proposal entitled “Further Clarifications on the Presented Evolved Architecture” will be discussed in the following in reference to FIG. 3.
The RNG (Radio Network Gateway) 301 is used for interworking with the conventional RAN, and to act as a mobility anchor point meaning that once an RNG 401 has been selected for the connection, it is retained for the duration of the call. This includes functions both in control plane and user plane.
On the control plane the RNG 301 acts as a signaling gateway between the evolved RAN and the CN, and the evolved RAN and R99/4/5 UTRAN. It has the following main functions:                Iu signaling gateway, i.e. anchor point for the RANAP (Radio Access Network Application Part) connection,                    RANAP connection termination, including:                            Setup and release of the signaling connections                Discrimination of connectionless messages                Processing of RANAP connectionless messages,                                    Relay of idle and connected mode paging message to the relevant NodeB+(s),                        The RNG takes the CN role in inter NodeB+ relocations,        User plane control and        Iur signaling gateway between NodeB+ 302-305 and R99/4/5 RNC.        
Further, the RNG is the user plane access point from the CN or conventional RAN to the evolved RAN. It has the following user plane functions:                User plane traffic switching during relocation,        Relaying GTP (GPRS tunneling protocol on the Iu interface) packets between NodeB+ and SGSN (Serving GPRS Support Node, an element of the CN) and        Iur interworking for user plane.        
The NodeB+ 302-305 element terminates all the RAN radio protocols (Layer 1—Physical Layer, Layer 2—Medium Access Control and Radio Link Control sub-layers, and Layer 3—Radio Resource Control). NodeB+ 302-305 control plane functions include all the functions related to the control of the connected mode terminals within the evolved RAN. Main functions are:                Control of the UE,        RANAP connection termination,                    Processing of RANAP connection oriented protocol messages                        Control/termination of the RRC (Radio Resource Control) connection and        Control of the initialization of the relevant user plane connections.        
The UE context is removed from the (serving) NodeB+ when the RRC connection is terminated, or when the functionality is relocated to another NodeB+ (serving NodeB+ relocation). Control plane functions include also all the functions for the control and configuration of the resources of the cells of the NodeB+ 302-305, and the allocation of the dedicated resources upon request from the control plane part of the serving NodeB+. The “+” in the term “NodeB+” expresses the enhanced functionality of the base station in comparison to the R99/4/5 specifications.
User plane functions of the NodeB+ 302-305 include the protocol functions of PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) and MAC (Media Access Control) and Macro Diversity Combining.
Another alternative for an evolved radio access network (RAN) architecture as proposed in “Radio Access Network Design Concept for the Fourth Generation Mobile Communication System” by Y. Yamao et al., Wireless Laboratories, NTT Mobile Communications Inc, is a cluster-cellular RAN structure is shown in FIG. 5. This architecture does not deviate from Rel99/4/5 architecture to such a large extent as the evolved architecture described above in reference to FIG. 3. The evolved architecture shown in FIG. 5 has a horizontal structure and localized handover processing.
The base stations (BSs) are grouped in clusters and there is a so-called cluster-head BS connecting the cluster to the RNC. BSs belonging to the same cluster are mutually connected through wired interfaces thus forming a sort of local area network (horizontal structure). Certain hierarchical differences in BS are introduced: the cluster-head BS is directly interfaced to the RNC and terminates Layer 3 signaling procedures for radio resource handling on multi-cell level.
Macro diversity gain due to handover is obtained by processing inside the cluster (localized handover processing). Most of the Layer 1/2 signaling is kept within a cluster thus reducing the loads on entrance links and RNC signal processing equipment. Each of the base stations may monitor channel quality autonomously and decides on whether it transmits a packet to a particular UE or not. This solution minimizes the probability of unnecessary or excessive transmission power and hence realizes high-efficiency and low-interference transmission.
HSDPA for UMTS
High Speed Downlink Packet Access (HSDPA) is a new technique that was standardized in UMTS Release 5. It may provide higher data rates in the downlink by introducing enhancements such as adaptive modulation to the Uu interface-between a UE and a Node B. HSDPA relies on Hybrid Automatic Repeat reQuest protocol (HARQ) Type II/III, rapid selection of users that are active on the shared channel and adaptation of transmission format parameters according to the time varying channel conditions. The unveiled invention is particularly applicable to HSDPA. Although most of the presented embodiments refer to HSDPA, the invention is not restricted to this system. Therefore the data transmission does not necessarily depend on a particular radio access scheme.
The User Plane Protocol Stack Architecture for HSDPA assuming Rel99/4/5 UTRAN architecture is shown in the FIG. 4. The HARQ protocol and scheduling is a function of the MAC-hs sublayer (MAC=Media Access Control) which is distributed across Node B and UE. An SR ARQ protocol based on sliding window mechanisms may also be established between RNC and UE on the level of Radio Link Control (RLC) sublayer in acknowledged mode. The service that is offered from RLC sublayer for point-to-point connection between core network (CN) and the UE may be also referred to as an Radio Access Bearer (RAB). Each RAB may be subsequently mapped to a service offered from MAC layer. This MAC-layer service may also be referred to as a Logical Channel (LC).
Parameters of the protocols may be configured by signaling in the Control Plane. In UMTS, the signaling between radio network (S-RNC) and UE may be controlled by the Radio Resource Control (RRC) protocol. The signaling among the UTRAN entities may be controlled by application protocols, e.g. the Node B Application Part (NBAP) on the Iub interface between Node B and RNC and the Radio Network Subsystem Application Part (RNSAP) on the Iur interface between RNCs.
The HS-DSCH FP (High Speed Downlink Shared Channel Frame Protocol) is responsible for flow control between Node B and RNC. It determines the capacity that can be granted to RNC for transmitting packets across the transport network based on requests obtained from RNC. More specifically, the capacity is requested by CAPACITY REQUEST messages of HS-DSCH FP originating from S-RNC. The permission for a UE to transmit a certain amount of data over a certain period of time is granted by CAPACITY GRANT messages sent from a Node B.
The format of a HS-DSCH FP data frame is shown in FIG. 6. The explanation of respective field is as follows. The data frame comprises a header, a payload section and a tail.
In the header the Header CRC field provides a Cyclic Redundancy Checksum calculated on the header of a data frame using a polynomial. Further, the header comprises the CmCH-PI (Common Transport Channel Priority Indicator) information element which indicates the priority of the data frame and the corresponding MAC-d PDUs. The CmCH-PI may be in the range 0 to 15, where 0 means lowest priority and 15 is the highest priority. The Frame Type (FT) field in the header describes if the present HS-DSCH FP data frame is a control frame or a data frame.
Further, the MAC-d PDU Length indicates the length in form of the number of bits of each MAC-d PDU in the payload of the HS-DSCH FP data frame. The Number of PDUs field Indicates the number of MAC-d PDUs in the payload section. The User Buffer Size field provides the users' buffer size (i.e. the amount of data in the buffer) in octets for a given Common Transport Channel Priority Indicator level.
Finally, the header comprises spare bits as indicated for future use.
In the payload section of a FP data frame comprises the MAC-d PDUs. A MAC-d PDU contains the C/T field of the MAC header followed by a RLC Packet Data Unit (PDU). The C/T field is used for providing an unambiguous identification of logical channels in case several logical channels are multiplexed on the same transport channel. Spare bits before the beginning of each MAC-D PDU in the payload section are also comprised. The payload section may comprise several MAC-d PDUs, wherein the total number of MAC-d PDUs in the payload section is indicated in the header (Number of PDUs).
In the tail section of the FP data frame a Payload CRC provides a Cyclic Redundancy Checksum calculated on the payload of a data frame with a polynomial. The Spare Extension indicates the location where new information elements (IEs) can be added in a backward compatible way for further extensions.
Mobility Management within R99/4/5 UTRAN
In this section some frequently used terms will be briefly defined and some procedures connected to mobility management will be outlined (see 3GPP TR 21.905: “Vocabulary for 3GPP Specifications” available at http://www.3gpp.org).
A radio link may be a logical association between single UE and a single UTRAN access point. Its physical realization comprises radio bearer transmissions.
A handover may be defined as transfer of a user's connection from one radio bearer to another. In contrast, during “soft handover” (SHO) radio links are established and abandoned such that the UE always keeps at least one radio link to the UTRAN. Soft handover is specific for networks employing Code Division Multiple Access (CDMA) technology. Handover execution is commonly controlled by S-RNC in mobile radio network.
The “active set” comprises a set of radio links simultaneously involved in a specific communication service between UE and radio network, e.g. during soft handover, the UE's active set comprises all radio links to the RAN's Node Bs serving the UE.
An Active set update procedure modifies the active set of the communication between UE and UTRAN. The procedure comprises three functions: radio link addition, radio link removal and combined radio link addition and removal. The maximum number of simultaneous radio links is set to eight. New radio links may be added to the active set once the pilot signal strengths of respective base stations exceed certain threshold relative to the pilot signal of the strongest member within active set. A radio link may be removed from the active set once the pilot signal strength of the respective base station exceeds certain threshold relative to the strongest member of the active set. The threshold for radio link addition is typically chosen to be higher than that for the radio link deletion.
Hence, addition and removal events form a hysteresis with respect to pilot signal strengths. Pilot signal measurements are reported to the network (S-RNC) from the UE by means of RRC signaling. Before sending measurement results, some filtering my be performed to average out the fast fading. A typical filtering duration may be about 200 ms and the duration contributes to handover delay. Based on measurement results, S-RNC may decide to trigger the execution of one of the functions of active set update procedure.
Specific Features of R99/4/5 HSDPA Architecture for Mobility Management
The R99/4/5 HSDPA Architecture is distributed in two different aspects: To the first aspect, entities of retransmission protocols transmitting on the downlink, RLC and MAC-hs, are located in the S-RNC and Node B respectively and, to the second aspect, radio resource management algorithms, handover control and packet scheduling are based on two independent measurements obtained from UE and are located in S-RNC and Node B respectively. These features have certain implications on mobility management and context preservation in HSDPA.
HS-PDSCH (High Speed Physical Downlink Shared channel) is a physical channel associated to HS-DSCH. The frame of the HS-PDSCH (TTI of 2 ms) are short compared to frames of dedicated channels (10 ms) in order to allow fast scheduling and fast link adaptation. Applying soft handover would cause problems in distributing scheduling responsibilities across all Node Bs of the active set and would require extremely tight timing to provide the scheduling decision to all members of the active set even if distribution of scheduling function were resolved. Therefore, soft handover is not supported for HS-PDSCH. Soft handover for A-DPCH is allowed which means it can be transmitted from more than one base station to a UE which combines obtained signals. A handover procedure related to HSDPA radio link is called serving HS-DSCH cell change.
During a serving HS-DSCH cell change procedure as shown in FIG. 7 and FIG. 8, the role of serving HS-DSCH link is transferred from the source radio cell 704 or link to a target radio cell 705. The two cells 704, 705 involved in the procedure are more specifically denoted source HS-DSCH cell and target HS-DSCH cell. The network-controlled serving HS-DSCH cell change is characterized in that the network makes the decision of the target cell. For example in UMTS Release 5, this decision process is carried out in the S-RNC 706.
Cell change procedure may also be initiated by the UE 703. In this case it is referred to as UE-controlled serving HS-DSCH change procedure. Another criterion for categorizing a cell change procedure is the categorization with respect to the serving HS-DSCH Node B.
The Node B 701 controlling the serving HS-DSCH cell 704 for a specific UE 703 is commonly called as the serving HS-DSCH Node B. Intra-Node B serving HS-DSCH cell change procedure is a cell change procedure wherein source and target HS-DSCH cells are controlled by the same Node B. In inter-Node B serving HS-DSCH cell change procedure, source and target HS-DSCH cells are controlled by different Node Bs 701, 702.
Synchronized serving cell change procedures are cell change procedures in which a Node B and a UE can simultaneously start transmitting/receiving signals after handover completion. Synchronization between the UE and the network is maintained with activation timers that are set by RRC entity in S-RNC. Due to unknown delays over Iub/Iur interfaces, processing and protocol delays, a suitable margin is assumed when determining activation timer setting. The margin also contributes to handover delay.
Executing an inter-Node B serving HS-DSCH cell change procedure also implies executing a serving HS-DSCH Node B relocation procedure. During the serving HS-DSCH Node B relocation procedure, a part of the MAC-hs protocol context may be lost.
In FIG. 7, the situation before the actual cell change of the UE 703 is shown. UE 703 communicates via a radio link of the source cell 704 with Node B 701, while RNC 706 decides to assign the source cell 705 of Node B 702. FIG. 8 illustrates the communication of the UE 703 using a new radio link of target cell 705 after having performed the cell change.
Assuming that the one way Iub delay equals 50 ms, the resulting worst case Node B buffer occupancies per user and for a specific service can be calculated (see table below). Depending on a specific flow control algorithm employed on the Iub interface, the Node B buffer occupancy can vary.
Service1.2 Mbps3.6 Mbps10 MbpsMean Node B+75002250062500buffer occupancy[byte]
Apart from handover delays that are specific for all procedures and result from measurement and synchronization delays, there is an additional delay introduced by this data loss. This delay is incurred due to compensation of lost packets.
For interactive services requiring high reliability of data transmission, end-to-end reliable transport protocols such as TCP are used. Compensation of lost packets by means of these protocols causes an additional delay mainly due to retransmitting the packets over core network and radio access network.
This increased delay may trigger spurious timeouts of timers of a reliable transport protocol (TCP) used for end-to-end transmission and may thereby slow down the data rate of packets being input to UTRAN due to congestion control mechanisms. This mechanism is for example described in “TCP/IP Illustrated Volume 1, The Protocols” by W. Richard Stevens, Addison-Wesley, 1994 (ISBN 0-201-63346-9). Assuming a TCP segment size equal to 1500 bytes, the amount of data lost due to the loss of the retransmission protocol's context in the Node B buffers is in the range from 5 to 41 segments. After performing cell change procedure it is probable that channel conditions of the user will be improved. However, due to invoked TCP congestion control a number of packets that are available for scheduling is decreased and radio resources are not utilized efficiently.